Postage meters that include rotary print drums for printing fixed and variable postage data on mail are already know. Particular mention may be made to French Patent Applications FR-A-89 16250 and FR-A-90 08752 filed in the name of the applicant.
For this type of meter, maximum overall postage meter compactness is sought, together with simplified electromechancial coupling between the various members making up the print head in the meter.
To facilitate understanding the problems posed by this type of meter, a prior art postage meter is described below with reference too FIGS. 1 and 2.
FIG. 1 shows an example of a print head 10. This print head 10 includes a print roller 12 secured to the end 14 of a shaft which can be mechanically coupled to a rotary drive device 16 in the meter. Inside the drum 12 there are print wheels 18 which have print elements 20 on their peripheries, which print elements correspond, for example, to all ten digits The print wheel 18 can rotate about its axis, as indicated by arrow F. Naturally, the drum includes as many wheels as there are graphical elements to be printed. The print wheel 18 is secured to a toothed wheel 22 so that it rotates therewith, which toothed wheel can be rotated by engagement with a rack 24 mounted at the end of a drive rod 26 which is in turn slidably mounted in the shaft 14 which is hollow. Each rod 26 associated with a wheel 18 is secured to a respective sliding ring 28 so that it moves in translation therewith. The ring 28 being displaced in translation enables the corresponding rack 24 to be controlled, whereby the print wheel 18 can be rotated to bring the suitable print element 20 to face a print window provided in the drum 12 and not shown in the FIGURE. The ring 28 may be displaced in translation by a slider 30 which co-operates with the ring 28 to move it in translation, but which leaves the ring free to rotate. A double rack 32 secured to the slider 30 co-operates with a toothed wheel 34 in turn secured to a knurled control wheel 36 for controlling the position of the print wheel 18. Adjusting the position of the control wheel 36 sets the position of the corresponding print element 20 so that it faces the print window. The other end of the double rack 32 makes it possible to rotate the wheel 38 of a position encoder bearing the overall reference 40. FIG. 1 also shows a printed circuit card 42 carrying the various electronic circuit components of the print head 10.
FIG. 2 is a plan view of the various rod-and-rack assemblies 24, 26 that are each associated with a respective position-control sliding ring 28.
The purpose of the position encoder 38 is to make sure that the position of the print wheel 18 actually does correspond to the position displayed by the control wheel 36.
Prior art print heads suffer from being bulky and complex due to the mechanical linkages between the control wheel 36, the position sensor 38, and the print wheel 18. These problems are proportional to the number of print wheels 18 required to print the values, dates, and other data, e.g. eleven wheels.
In prior embodiments, one rack is used for each print wheel. Each rack performs the following three functions:
A first end of the rack co-operates with the print wheel to define the digit or the symbol to be printed. A second end of the rack serves to position the rack, and therefore the corresponding print wheel, by means of a motor which receives instructions in electrical form. A middle portion co-operates with the position encoder to read back the effective position of the rack, and therefore of the print wheel.
The use of racks suffers from two types of drawback.
Firstly, the rack can only have reciprocating motion and cannot have continuous motion. As a result, if it is necessary to print a zero with a wheel, after that wheel has printed a 9, the associated rack must be displaced over its full stroke. This significantly increases the basic print time required to print one digit.
Secondly, since the position encoder is disposed along the rack, the revolving electronic circuitry must be disposed parallel to the shaft 14. If the quantity of electronic components increases, that length of the shaft 14 which serves only to support the printed circuit also increases. This leads to an increase in the overall length of the shaft.
An object of the present invention is to provide a postage meter having a rotary print drum that does not suffer from these problems, and that offers maximum integration of the print head.